1. Field of the Invention
The present invention relates to a liquid crystal display device with a color compensation function by having liquid crystal molecules oriented by twisting 90 degrees or more (supertwisted) as a display medium.
2. Description of the Prior Art
Generally a supertwisted nematic (STN) liquid crystal display device is colored in yellow-green or blue color, but the color is corrected by using an optical compensating plate and a bright and sharp black/white display is obtained. Accordingly, the display quality is enhanced, and it can be used as the display element for a word processor, a computer and other business machines.
As the color-compensated supertwisted liquid crystal display device, a two-layer supertwisted liquid crystal display device is known well, in which the coloring occurring in the first layer (a driving panel) is corrected in the second layer (optical compensating panel) to realize a colorless display. This structure requires two liquid crystal panels, as compared with the single-layer supertwisted liquid crystal display device, and the thickness of the display device increases, which adds to the weight. To solve this problem, using a phase difference plate composed of a uniaxially drawn polymer film as the optical compensating plate, a thin and light-weight supertwisted liquid crystal display device has been developed. This phase difference plate is, however, manufactured by drawing a polymer film, and differs in the optical properties between the film drawing direction and its orthogonal direction, and therefore as compared with the two-layer supertwisted liquid crystal display device, the color change due to the azimuth or elevation angle is larger in the supertwisted liquid crystal display device of phase difference plate type; that is, the viewing angle is narrower.
It is because of its optical anisotropy that a uniaxially drawn polymer film is used as a phase difference plate. That is, between the polymer film drawing direction and its orthogonal direction, the refractive index is different (birefringence). The retardation (.DELTA.n.d) given by the product of this refractive anisotropy .DELTA.n and the film thickness d is a physical quantity that brings about phase difference of the light occurring when passing through the film, and the change of this due to elevation angle differs between the drawing angle and its orthogonal direction. For example, in a phase difference plate made of polycarbonate, as the elevation angle becomes larger, the retardation decreases in the drawing direction and increases in the orthogonal direction. As a result, when combined with a liquid crystal display panel, if the optical compensation relation is perfect in the normal direction, as the elevation angle becomes larger, the difference between the retardation of the phase difference plate and the retardation of the liquid crystal panel becomes larger, and the optical compensation relation collapses. In other words, a color change occurs, and the contrast ratio is lowered, and therefore the viewing angle is narrowed.